1. Field of the Invention
The present invention relates to a decoupling capacitor used in power supply circuits for electronic devices and more particularly to a solid electrolytic capacitor having a plurality of mounting terminals in particular.
2. Description of the Related Art
In recent years, electronic devices have been made small-sized, thin, and highly functional in a progressive manner and one of efficient means to achieve the progression is to make a circuit driving frequency high. To realize this, a reduction of equivalent series inductance (hereafter simply “ESL”) is becoming a recent rend.
An increase in ESL is attributable to factors including permeability of a conductor in a device, wire length from an inside portion of the device to a mounting terminal, shape of the wire, or a like. To solve these problems, various methods have been available recently including a method in which anode and cathode mounting terminals are made to come near to each other to decrease an inductance component called loop inductance occurring between the anode and cathode terminals and a method in which the number of the mounting terminals is decreased and the anode and cathode terminals are disposed alternately and one-dimensionally or two-dimensionally in a staggered format or in a zigzag form.
For example, a capacitor structure is disclosed in Patent Reference 1 (Japanese Patent Application Laid-open No. 2002-343686) in which ESL is made small by alternately disposing an anode mounting terminal and cathode mounting terminal and, in order to realize the alternate disposition, after forming an insulating resin portion on a parent metal in an element portion and on a porous substance portion obtained after the formation of a solid electrolyte, by forming through holes in the insulating resin portion and by filling inside portions of the through holes with conductors, cathode conductors in the element portion is connected to the cathode mounting terminals and an anode mounting terminal is formed directly on the parent metal in the element portion. Moreover, the surfaces on which the anode mounting terminal and cathode mounting terminal are disposed are coated with an insulating protecting layer (sheathing resin).
Also, another capacitor structure is disclosed in Patent Reference 2 (Japanese Patent Application Laid-open No. 2002-289469) in which zigzag-shaped disposition of terminals is realized without forming through holes in a parent metal. Here, an electrode portion made non-porous is formed on one side of a sheet of valve-action metal making up a parent metal and a connecting terminal to be connected to the electrode portion is used as a positive mounting terminal. After portions of the valve-metal sheet other than the electrode portion are made porous, a dielectric film is formed in the portion and further a solid electrolyte layer and current collector layer are formed on the dielectric film and a connecting terminal to be connected to the current collector layer is used as a negative mounting terminal. The anode mounting terminal and cathode mounting terminal are insulated by an insulator being interposed between the anode and cathode mounting terminals.
However, conventional capacitor structures have problems. That is, in the conventional solid electrolytic capacitor disclosed in the Patent Reference 1, after forming a required number of through holes in the porous substance portion, by filling the through holes with the insulating resin and curing the filled resin, and then by forming the second through holes with a diameter not exceeding the diameter of through holes formed in an initial stage at center portions of the cured resist portions and further by coating and filling the inside portions of the through holes by plating, the conductors are obtained. However, there is a problem that a leakage current characteristic of the solid electrolytic capacitor is degraded due to cracking occurring in the resin portions by mechanical and thermal stresses or due to damage caused in the porous portion in portions surrounding the through holes while the second through holes are being formed.
Moreover, in the conventional solid electrode capacitor disclosed in the Patent Reference 2, since no through holes are formed in the porous substance portion, the above problems occurred in the solid electrolytic capacitor disclosed in the Patent Reference 1 can be solved, however, since the anode and cathode mounting terminals are formed directly on the parent metal and on the porous substance portion of the parent metal, there is a problem that, when the solid electrolytic capacitor is mounted on the substrate or in the temperature cycle, the characteristics of the solid electrolytic capacitor, equivalent series resistance in particular is degraded by thermal stress occurring due to a difference in thermal expansion coefficients between the substrate and the solid electrolytic capacitor.